Just reading around and it seems that some / many / most (no idea which) induction hobs are fitted with power factor correction. It's not the kind of thing that's generally published in the sales brochure so best clarify with the manufacturer before specifying it.

Regarding lighting, if you wish a 48 volt battery but a lower voltage for lighting, and would prefer to avoid the complication of a 3 wire DC system, it might be worth considering running the lighting at 12 or 24 volts DC from a DC/DC converter.
This has some of the drawbacks of an inverter in that extra losses and another point of failure are introduced.

OTOH, DC/DC converters are now much reduced in price and improved in efficiency.
The lighting load could be divided between two converters so as to avoid a total blackout if one fails. Keeping a spare or two would be very affordable.

12 volts is more viable since the converter may be situated close to the centre of the load. The converter limits the fault current so cheap automotive fuses may be used for the lighting circuits._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

Sounds good.
Just in case the converter fails it might be worth installing a handful of lamps in key locations that are powered direct* from the battery.

As these wont be regularly used, low efficiency is fine and incandescent lamps may be used. Just 6 lamps in key locations, each 40 watts would be ample for emergency lighting.
Incandescent lamps supplied direct* from the battery should still work after an EMP event that has killed anything more sophisticated.

*via a fuse of course._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

I have just received information on the latest multi voltage LED lamps.

7 watt, warm white, and said to be suitable for "12, volt, 24 volt, 48 volt and 60 volt systems"
Should also be suited for other voltages within that range such as 16 volts or 32 volts.

Rather expensive at over £20 each, but will probably become cheaper in time.

Available from international lamps. This is the first time that I have seen these stocked by a major and well known wholesaler, rather than by obscure internet based outlets._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

Will you require electric lights in any relatively distant outbuildings ? or outdoor lighting for any night time work ?

If so, then consider use of 50 volt lamps for these lights in order that cheap thin cable may be used.
The multi voltage lamps to which I linked earlier would be ideal, the high cost of the lamps being offset by reduced cable costs.

The main limitation on the use of 50 volts DC is the lack of light switches suitable for 50 volts DC.
All modern light switches are marked "AC ONLY" but in practice are fine on DC provided that the voltage does not exceed about 10% of the AC voltage rating.
I would use a modern light switch on 24 volts DC but not on 50 volts DC.

Options include searching fleabay for genuine old light switches (not modern repro ones) that were manufactured in the days of DC mains and are marked "AC/DC"

Or use of a modern heavy duty 2 pole switch and wiring both poles of the switch in series, or looking for switches rated at 500 or 690 volts AC_________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

Our barn is some 80m from the house, but has its own solar system. I recently acquired 30m if ridiculously heavy gauge (the roll weighs around 50kg) armoured three core cable so if I ever did run power to our (yet to be built) greenhouse I'd use that.

Maybe question for Adam2, with my inverter, I can select 230V or 240V. I'm currently running at 230V (and it is always exactly 230V) and everything works fine - is there any reason/benefit to run at 240V instead? Or might it cause problems if I ever come across some old European device designed for 220V that won't like 240V?_________________PowerSwitch on Facebook | The Oil Drum | Twitter | Blog

The choice of 230 volts or 240 volts probably does not much matter.
Remember that public electricity supplies vary a lot.
In the UK, the old standard was 240 volts plus or minus 6%, so from 226 volts up to 254 volts, and that was at the meter, allowing for voltage drop within the premises, the voltage at the point of use could vary from about 212 volts up to 254 volts.

The new UK standard is 230 volts, plus 10% or minus 6% so that is from 217 volts up to 253 volts, again at the meter. At the point of use, the voltage could vary from 255* volts down to about 205 volts.

In mainland Europe the voltage was formerly 220 volts with a tolerance that varied a lot.

I might be inclined to select 240 volts at the inverter, on the grounds that whilst UK public supplies vary as stated above, most are in practice nearer to 240 volts than to 230.
240 volts would be preferable for power tools connected via long and perhaps marginally sized extension leads.

*That is not a typo, these days we have to allow for a slight voltage RISE of about 1% due to grid tied PV installations._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

I've finally got around to designing the solar electric heating system, a way to dump excess PV energy into hot water by way of programmable relays in 48V DC immersion heaters.

Any reason this wouldn't work - any improvements?

The Challenge
To ‘dump’ as much excess solar photovoltaic energy into hot water tank as possible whilst minimising impact on battery store.

Background
Our house is off-grid, powered by 6.5 kWp solar photovoltaics and a wood stove with back boiler. Being off-grid, we can’t export electricity so it either has to be used as it’s being generated or stored in batteries. Once the batteries are full, it’s a case of use it or lose it. We want to store this otherwise lost energy as heat our hot water tank.

First question AC or DC. I think it’s better to use 48V DC immersion heater(s) so as not to use the inverters unnecessarily or even keep them on all the time (we have a smaller 1.2 kVA inverter which is on all the time). The thermal store is only about 10m from the battery store so cabling is not unreasonable.

Relays
The MPPT, BMV and CCGX all have relays with the following options available:

BlueSolar MPPT

BMV-702

Colour Control GX
Has a relay, but can only work on information provided by the other devices so maybe there’s no point in using it? However, it does have a neat way to start/stop a generator, track hours run etc. I wonder if this assistant could be used to start/stop the immersion heaters?
https://www.victronenergy.com/live/ccgx:generator_start_stop

I don’t yet own the 48V immersion heaters, but say I get 3x 1.5kW for 4.5kW total. These could take a single cable then be hooked up in parallel at the water tank. This would be around 90A, over 10m so 14mm2 (6 AWG) for a 5% voltage drop. That’s some thick cable, and a single on/off or 4.5kW is not very delicate! Maybe better to run a separate cable to each immersion (more manageable 4.5mm (11 AWG) and have the choice of 1.5, 3 or 4.5 kW depending on how much excess solar was incoming.

Questions
Which relay(s) to use?
What to trigger on?
Relay minimum close time?
Is there a way of varying the power to the heaters, such that if there’s only 500W of ‘spare’ solar, that’s all that’ll get dumped?

Current thinking:
Proposal, use voltage. Have three 1.5 kW (or 1kW) immersion heaters on separate relays and cables.

First from the BMV using ‘default mode’ and high voltage thresholds 20 and 21 set such that the relay closes when the voltage reaches 56V (likely during bulk charge) and opening again if the voltage drops to 51.2V (equivalent to 12.8V on a 12V system indicating that the load is on the battery rather than being met from the PV). Set option 14 (minimum close time) to something like 5 minutes to reduce rapid switching.

Second from the one of the MPPT charge controllers, option 7 and thresholds 14 and 15 such that the relay closes at 56.3V (slightly higher so it doesn’t come in at the same time as the first one) and off at the same 51.2V, again with a minimum close time of 5 minutes.

Third from the other MPPT with either identical settings or a slightly higher relay close voltage if that’s possible given they the two are synchronised and the manual says settings have to be identical.

Good points: When the batteries are well on the way to being full the immersion heaters will switch on one after another. If there isn’t enough ‘surplus’ solar energy coming in, this load will be partially met from the batteries pulling the voltage down below 51.2V and disconnecting the load (after a few minutes).

Bad points: There has to be at least 1.5 kW (or 1kW) spare energy in order to put anything in the water. If there’s only a 500W surplus, that can’t be used. This configuration requires three separate solid state relays and they aren’t cheap._________________PowerSwitch on Facebook | The Oil Drum | Twitter | Blog

Last edited by clv101 on Sat Feb 24, 2018 9:21 am; edited 1 time in total

If you only have 500w you connect 3 x 48v elements in series then connect that across whatever 48v supply you were expecting to drive at 1.5k/3k/4.5k

You could also connect 2 elements in series for 750w, but the logic switching will become messy. It could be controlled by low level logic, but it would need to be a failsafe arrangement as the consequences of failure could be big and smokey. Presumably if you place too heavy a load on the panels eg ~4.5kW
on a dull day, you just lose efficiency - or is there other issues?

Probably easier to use an 8kW mains shower element at 48V for 320W. Not sure how you are going to reliably switch very heavy currents.

On strictly electrical grounds, you are correct, but in practice it is not a good idea. Electric shower elements are designed to heat continually flowing water unlike standard immersion heaters that merely require to be submerged in water.
A shower element is not really suited for fitting within a water tank, and if fitted external to the tank would a circulating pump, adding complexity._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"